Chemistry - A European Journal

The second generation of chiral bases: this paper highlights recent advances and new concepts in the chemistry of chiral bases. Complementary to chiral lithium amides, which are known since the end of the eighties, new classes of chiral bases (chiral magnesium bisamides and chiral alkali alkoxides) are emerging, such as depicted, giving access to new applications of chiral bases in asymmetric synthesis.

Novel macromolecular architectures including dendritic and star-shaped polymers and organic nanoparticles are synthesized based on a modular approach from several libraries of building blocks including functional dendrimers (see picture) and dendrons. One of the distinctive features of these well-defined three-dimensional macromolecules is to facilitate nanophase morphologies in mixtures with organosilicates and ultimately, upon thermolysis of the organic component, nanoporous organosilicate networks.

A new methodology for synthesis of polymer–polymer inclusion complexes is described. This is achieved by an amylose-forming polymerization in the presence of appropriate synthetic guest polymers (see scheme). The reaction system proceeds in a similar way to which vines of plants grow twining around a rod; therefore, this process has been named as “vine-twining polymerization”.

The highly stereoselective, intramolecular formation of the aryl–aryl bond is the key step of an efficient strategy for the synthesis of atropisomeric diphosphines, such as (S)-MeO-NAPhePHOS (see picture), bearing heterotopic aryl moieties.

A photochromic system involving an interconversion between a coronand and a cryptand operates through a reversible intramolecular photocycloaddition reaction between two anthracene units (see scheme). The coronand binds Group 1 cations (M⁺) more strongly than the cryptand in methanol solution.

Spacer units that impose trans coordination on rhodium and iridium centers are present in newly designed diphosphine ligands. The complexes thus obtained efficiently catalyze the carbonylation of methanol and allow, in the case of complex 1, the recovery of the unchanged catalyst.

A donor-acceptor-donor triad molecule (OPV-PERY-OPV) has been synthesized which comprises a perylene bisimide derivative as electron acceptor, and an oligo(p-phenylene vinylene) (OPV) derivative as electron donor. A simple model compound (Ph-PERY-Ph) was also synthesized in which the OPV units were replaced by phenyl groups. In polar solvents these two compounds are present as dimers, that is, two perylene molecules are held together by intermolecular hydrogen bonding with the perylene core shifted slightly with respect to one another.

Fluorescent-labelled lipopeptides such as a characteristic S-palmitoylated partial structure from influenza virus A hemagglutinin (1) can be synthesized efficiently by employing a new enzymatic protecting group technique in the key steps. The binding to model membranes was determined in a kinetic assay leading to a first approximation of the membrane-anchoring ability of the corresponding lipopeptide motif in the parent protein.

Almost quantitative formation of the tetrameric macrocycle [{P(μ-NtBu)}₂(μ-NH)]₄ (3) together with a trace amount of the pentameric host–guest complex [{P(μ-NtBu)}₂(μ-NH)]₄(HCl) (4) occurs from the reaction of the dimers [ClP(μ-NtBu)]₂ (1) and [NH₂P(μ-NtBu)]₂ (2) in THF/Et₃N (see scheme). Amplification of the amount of 4 occurs in the presence of halide ions.

The first structurally characterized η⁴coordinated S₈rings are observed in the essentially C_4v symmetric [AgS₈]⁺ (left) and centrosymmetric [AgS₁₆]⁺ (right) cations that were stabilized by very weakly coordinating counterions of type [Al(OR)₄]⁻ or [Sb(OTeF₅)₆]⁻. The solid-state geometries of the [AgS₈]⁺ and [AgS₁₆]⁺ cations are close to but not identical to those calculated with MP2 and their geometries are the best approximation of the structure of these gaseous ions observed earlier by MS methods.

The relative aromaticities of the singlet benzyne isomers, 1,2- (1), 1,3- (2), and 1,4-didehydrobenzene (3; o-, m- and p-benzyne, respectively) are evaluated by the use of a series of aromaticity indicators, including nucleus-independent chemical shifts, magnetic susceptibility exaltations and anisotropies, aromatic stabilization energies, and valence-bond Pauling resonance energies.

A carboxyl and a carbonyl group can be added enantioselectively to the vicinal carbon atoms of an olefin by a two-step sequence as depicted. The sequence works very well (good yields and high ee values) for cyclic and cis-1,2-disubstituted acyclic olefins.

B3LYP and MP2 calculations show that the high endo selectivities of hetero-Diels–Alder reactions of ortho-xylylenes with substituted aldehydes are a result of CH–π or attractive π–π interactions between the aldehyde substituent and the ortho-xylylene π system (see picture).

Organotin compounds grafted onto cross-linked polystyrene beads (see scheme) have been investigated by using 1D and 2D ¹H, ¹³C, and ¹¹⁹Sn hr-MAS NMR spectroscopy, which demonstrates the potential of this method to characterize organometal compounds directly at the solid/liquid interface.

Weak intramolecular antiferromagnetic (Ln=Dy, Er) or ferromagnetic (Ln=Ce, Nd, Yb) interactions do exist in (Gd, Ln) complexes. This is the key finding of magnetic studies on heterodinuclear (Gd, Ln) lanthanide complexes (e.g. LGdLa(NO₃)₃⋅2 H₂O), which were synthesized in a novel stepwise procedure by using the tripodal ligand tris[4-(2-hydroxy-3-methoxyphenyl)-3-aza-3-buten]amine (LH₃). The plot shows the Δχ_MT difference between the (Yb, Gd) and (Yb, La) complexes.

Heterocyclic helicity codons provide the basis for conformational control and thereby for a strategy for designing folding properties into synthetic oligomers. Strong intermolecular interactions of the highly ordered lock-washer subunits of compound 1, which contains an alternating pyridine–pyridazine strand comprising thirteen heterocycles, result in hierarchical supramolecular self-assembly into protofibrils and fibrils. Compound 1 also forms mechanically stable two-dimensional Langmuir–Blodgett and cast thin films.

Ligands containing three terpyridine-like subunits react with metal ions such as Zn^II, Co^II and Hg^II to give two [3×3] and [2×3] grid-type coordination architectures (see picture). Suitable design of ligand structure and appropriate choice of reaction parameters allows the generation of metal complexes of well-defined geometry and nuclearity presenting attractive features as functional inorganic arrays.

Stepwise rather than concerted: both the optically and thermally induced electron transfer in the trication of the highly symmetrical (D₃) hexaarylbenzene system 1 proceed not by a concerted three-electron transfer pathway but instead by three consecutive transfer steps.

In order to clarify the solvation mechanisms responsible for their partition, the lipophilicity of 28 acids, such as Ibuprofen (1), has been studied by potentiometry and cyclic voltammetry in two-solvent systems. Results are interpreted in terms of intramolecular interactions and conformational effects in the neutral and anionic forms. Born's solvation model describes qualitatively the effect of charge delocalization in the anion.

In contrast to what might be expected the “N-4 in” isomers of porphyrin, even though they have very different core shapes (see picture), act as surprisingly good ligands for a variety of metal cations.

The challenge of rational synthesis of metallocarbohedrenes (depicted here: [M₈(L)₈(C₂)₆] M from Groups 4–10; L=NR₃, PR₃, Cl⁻, O²⁻, CN⁻, Cp⁻, CO) known only in the gas phase is addressed with density functional prescriptions of electronic and geometric structure: 38 out of 150 molecules investigated are prescribed as favourable targets, with synthetic reactions proposed.

A charge density study is described that uses single-crystal CCD X-ray diffraction data on C₆H₆ at T≈110 K combined with anisotropic displacement parameters (ADPs) for C and H atoms derived from the temperature dependence of ADPs for C and D atoms in C₆D₆ obtained by neutron diffraction. The derived quadrupole moment of the molecule in the crystal and average deuterium nuclear quadrupole coupling constants are in excellent agreement with independent measurements. An electron density map of benzene is shown here.

What is the energetic basis for enantioselection by cyclodextrins? An answer to this question in a simple host–guest system, the 1:2 complexation of camphor by α-cyclodextrin in water (see picture), is sought from microcalorimetric binding studies and solvent isotope effects.

More surprises in the P–N binary phase diagram: we investigated structural and vibrational properties of α-P₃N₅ and γ-P₃N₅ with density functional methods. A modeling of pressure revealed that above 40 GPa, yet another high-pressure modification with a kyanite-type structure might appear. This modification, denoted δ-P₃N₅, exhibits {P}N₆ octahedra (see picture), which are unprecedented in phosphorus–nitrogen chemistry.

Triangle–square equilibria, which are dependent on the flexibility of the ligands connecting the metal ions (see, for example, the supramolecular square complex depicted), can be observed by ESI-FT-ICR mass spectrometry and by NMR spectroscopy. Gas-phase experiments uncover a supramolecular “neighboring-group” effect which favors the formation of triangles from squares over any other fragmentation pathway.